Recently, researches are being widely made on illumination devices or image display devices using solid-state luminescent devices, such as semiconductor light-emitting diodes. From among such illumination devices, a while light-emitting diode (LED) lamp emits light as electrons and holes are combined with each other and is being spotlighted as a next generation highly-efficient illumination device to replace incandescent lamps and fluorescent lamps in the prior art. Advantages of the white LED include reduced energy consumption due to its efficiency, exclusion of materials with high environment burdens, such as mercury, miniaturization of lamps for installation in a narrow space, high vibration resistance, and long lifespan. Therefore, a white LED may be applied to various purposes.
A white LED lamp in the prior art includes a LED device emitting a blue short wavelength light and a phosphor which emits a yellow long wavelength light by being excited by absorbing the blue short wavelength light partially or entirely. In this case, white light is obtained as a blue light emitted by the blue LED and a yellow light emitted by the excited phosphor are mixed. An example of yellow light emitting phosphors in the prior art is a garnet-based phosphor consisting of cerium-activated yttrium and aluminum. In this case, white light may be obtained by mixing a blue light emitted by a blue LED chip around 450 nm and a yellow light emitted by a yellow phosphor around 560 nm. However, a white light obtained as described above includes insufficient red component, and thus it is difficult to improve color rendition. Furthermore, due to the lack of red component, it is difficult to obtain a white illumination device having a sufficiently low color temperature similar to that of a light bulb.
To obtain an illumination device with improved color rendition and a low color temperature, a yellow phosphor may be mixed with a small amount of red phosphor, or a green phosphor and a red phosphor may be used instead of a yellow phosphor. In this case, it is necessary to develop a red phosphor with excellent color purity. However, since a red phosphor normally has a relatively low excitation spectrum in ultraviolet ray region or blue region, the red phosphor features lower efficiency than a green phosphor or a blue phosphor. Furthermore, it is necessary to mix a red phosphor in an entire phosphor at a relatively large ratio (60 wt % or above). Furthermore, to guarantee long lifespan and high output power to an illumination device and a display device, a red phosphor needs to have excellent temperature stability.